1. Field of the Invention
The invention relates to a microinverter primarily, though not exclusively, intended for use in a solar power installation and specifically one powered by a photovoltaic solar panel.
2. Description of the Prior Art
Currently, an increasing worldwide emphasis is being placed on exploiting clean, renewable energy sources rather than fossil fuels. One such energy source that is receiving considerable attention in the marketplace is solar energy. It is readily abundant, weather permitting, in a wide multitude of locales and across widely differing climates.
In essence, an amount of solar energy is harvested and converted into electrical power which, in turn, is either used to power local loads and/or fed to a power (utility) grid for consumption at remote locales from the point at which the energy was harvested. To do so, a matrix of inter-connected photovoltaic elements, called a “solar panel”, is aimed at the sun and converts incident solar radiation into a direct current (DC) output. Oftentimes, this DC output is converted through a companion microinverter, into appropriate alternating current (AC) line power, to provide an electrical power source for powering local, line-powered devices and/or for supplying the resultant AC power as input to a utility grid. The microinverter generally employs internal DC-to-DC converter and chopper stages with the DC-to-DC converter being used to convert, with ideally relatively low loss, the DC output voltage produced by the solar panel to a level suitable for efficient conversion to an power-line AC level, such as 120 volts at 60 Hz.
Oftentimes, a solar power installation utilizes individual assemblies of one or two panels that are mounted to a racking system. Each panel has an array of serially-connected photovoltaic (PV) cells. The output of the panels in each single assembly is connected to a microinverter which, itself, is mounted to the assembly and situated directly behind the racking system. Each such assembly has rather limited space to accommodate its corresponding microinverter.
Some conventional microinverters which are commercially available in the marketplace often utilize a design such as that described in an application report: “TMS 320C2000 DSP Controllers: A Perfect Fit for Solar Power Inverters”, Texas Instruments Application Report SPRAE3-May 2006, pages 1-8 and particularly FIGS. 3 and 4 on page 4 thereof. Unfortunately, this design suffers from various drawbacks which tend to limit its utility and attractiveness.
First, this design relies on using one or more, and often two, inductors, operating at a relatively high frequency, as primary internal energy storage devices, with the stored energy typically being routed by a steering diode into a capacitor. Unfortunately, the stored energy is not regulated particularly well and also operating an inductor at such a frequency causes increased electrical loss. Further, since the energy capacity of an inductor is limited by its physical size, this design does not scale well in terms of its own overall physical size. Specifically, the overall size of such a microinverter is very dependent on the size of the inductors. While suitable inductors for use in a microinverter that generates approximately a hundred watts of output power are sufficiently small, those for microinverters handling several hundred watts and more can be rather large thus, in turn, forcing the microinverter to have a corresponding large overall size. As the overall size increases, use of such microinverters becomes increasingly impractical for certain applications and simply unsuited for installation in space-limited solar panel assemblies. Moreover, the inductor-based design tends, relatively speaking, to exhibit poor output regulation and poor efficiency.
Second, this design relies on using two high-frequency choppers, rather than one, thus causing increased electrical switching losses. Further, the second (output) chopper is formed of an H-bridge, of four FETs (field effect transistors). The FETs are controlled through pulse width modulation (PWM) to form a sine wave output from input DC power. By virtue of high frequency switching under PWM control, each FET incurs switching losses, and dissipates several watts of power (e.g., 7 or 8 watts, when providing 170 watts of output power), thus further decreasing the overall efficiency of the microinverter. Moreover, the output of the FETs are basically coupled (though typically through an inductor and other components) directly to the AC utility line. Consequently the FETs, during their OFF-to-ON and ON-to-OFF transitions, are quite sensitive to and can be readily damaged by source-drain over-voltage conditions caused by transients existing on the utility line. This, in turn, may lead to premature device failures and hence compromise long-term reliability of the microinverter.
In this and similar designs, the two high frequency chopper stages must be preceded by an energy storage capacitor. These are typically large value aluminum electrolytic capacitors, generally viewed as a weak link in reliability. The capacitors source pulse currents demanded by their associated chopper transistors. The capacitor preceding the second stage inverter has an additional burden of absorbing pulse currents from the output of the first inverter stage. High current and high temperature (exacerbated by self-heating) appear to be key factors which degrade capacitor lifetime and overall system reliability. Hence, in the conventional designs, electrolytic capacitors are used twice. Further, Voltage transients on the utility grid can pass through the conducting H-Bridge FETs and into the energy storage capacitor. Since a large value capacitor presents a relatively low impedance, voltage transients from the utility grid tend to result in high current transients within the output stage. Stress caused by repeated high currents tends to reduce overall system reliability.
Furthermore, utilities require that, for connection to the grid, microinverters need to achieve as close to a unity power factor as realistically possible. To do so, conventional microinverters, such as that taught by the Texas Instruments SPRAAE3 application report and others like it, control their output voltage to track the grid voltage. Unfortunately, the grid voltage often contains distortions, be it transients, glitches, spikes or other undesired short-term variations, from a clean sinusoidal waveform. As such, the output of those microinverters will be controlled, via PWM, to similarly track those distortions, thus disadvantageously producing an output voltage that effectively reinforces their occurrence. Further, these designs often include both positive and negative feedback loops which, in turn, tend to make those units susceptible of breaking into oscillation and hence causing unstable operation.
Moreover, various conventional designs do not include galvanic isolation and thus present possible safety issues, hence potentially frustrating certification through certain testing and certifying organizations.
Therefore, a need exists in the art for a microinverter primarily, though not exclusively, suited for use in solar power applications that does not utilize an inductor as a primary energy storage device, and that exhibits relatively high and increased overall efficiency, stability, robustness and reliability over conventional designs. Such a microinverter should not produce an output that reinforces distortions appearing on the utility grid and should implement galvanic isolation. Ideally, such a microinverter should also be readily scalable to handle increased power levels without significant corresponding increases in its overall physical size.